The technical part of the profession of the optician consists in fitting a pair of ophthalmic lenses to a frame selected by a wearer. This task comprises five main operations:                acquiring framing parameters, notably geometrical data of the bezels of the rims of the frame selected by the wearer, in particular the curve traced out by the bottom of the bezel, and, where appropriate, the shape of the section of the bezel;        centring each lens, i.e. determining the position that each lens will occupy in the frame in order to be correctly centred in front of the pupil of the eye of the wearer so that it performs correctly the optical function for which it was designed;        using a feeler system to determine the coordinates of points on each face of each lens characterizing the geometry of its contour after trimming, then        producing setpoint trimming radii (in the form of what is called a radii reproduction function) that defines the intended spatial position of the contour on the lens to be trimmed taking account of the geometrical data read from the corresponding bezel, the defined centring parameters, and the coordinates of the lens determined by the feeler system, and finally        trimming each lens, i.e. machining or cutting its contour to the required shape, taking account of the defined setpoint trimming radii, where appropriate, at the end of machining, using a bevelling grinding tool for bevelling, i.e. producing a bevel on the edge of the lens intended to retain the lens in the bezel of the frame.        
To simplify and to improve the quality of the work carried out by the optician, it is desirable for the machining to be sufficiently precise for the lens to nest exactly and without force in the bezel of the frame chosen by the wearer. This precision avoids the optician being obliged to trim and bevel the lens repeatedly in order to fit it to its frame.
To obtain very high precision it is possible to determine the geometrical data of the bezel, including the shape of its section, at numerous points on the contour of the rims of the frame. Such measurements are time-consuming and wearisome, however.